In thermodynamics, Ebullioscopic constant relates molality to the boiling point elevation.

This relation is

Where is the elevation in the boiling point, i is the van’t Hoff factor, b is the molality and K_b is the Ebullioscopic constant.

Thus, the unit of an ebullioscopic constant is K (molality)^-1 or K Kg mol^-1, as molality= .

Thus, Option (i) is the correct answer.